1. Field of the Invention
The present invention relates to an oxide superconductor and a method of manufacturing the same.
2. Description of the Related Art
High critical-current oxide superconducting materials which are recently beginning to be put into practical use are expected to be usefully applied to, e.g., a fusion reactor, a magnetically levitated train, a particle accelerator, and a magnetic resonance imaging apparatus (MRI), and some materials are already put into practical use.
Major oxide superconductors are bismuth-based and yttrium-based (to be referred to as Y-based hereinafter) superconductors, and the Y-based superconductors having good magnetic field characteristics are attracting a great deal of attention as materials which will be put into practical use in the near future. The Y-based superconductors are oxides represented by YBa2Cu3O7-x, and oxides having a structure in which yttrium is substituted with a lanthanoide series element are also known as superconductors having good magnetic field characteristics. Known examples of the lanthanoide series elements are lanthanum, neodymium, samarium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, and ytterbium.
Examples of a method of preparing a Y-based superconducting film are pulsed laser deposition (PLD), liquid phase epitaxy (LPE), electron beam (EB) processing, and metalorganic deposition (MOD). Of these methods, non-vacuum, low-cost MOD is being spotlighted in recent years and extensively studied mainly in the United States of America and Japan. It is recently reported that among other MOD methods, MOD using trifluoroacetates (referred to as TFA-MOD hereinafter) can prepare a superconducting film having good properties.
MOD is a method comprising: coating a single-crystal substrate with a chemical solution by spin coating or dip coating, followed by drying the chemical solution to form a gel film, and performing heat treatments twice under normal pressure for the gel film, i.e., calcining and firing, to prepare a superconducting film. In this method, an oxide is formed by decomposing organic materials contained in a precursor by calcining, and a biaxially oriented structure is formed in the oxide layer by firing performed in a range of 700° C. to 900° C.
MOD has a problem that crystallites are formed after calcining, a disordered oriented structure is formed from these crystallites during firing, and the influence of the phenomenon particularly increases when the thickness is 100 nm or more. To provide a highly oriented structure by this method, it is important to perform rapid heating and rapid cooling within a short period of time so that no crystallites are formed by crystal growth of the thermally decomposed oxide in the calcined film. The rapid heating and rapid cooling are performed by loading and unloading a sample into and from an electric furnace. However, it is difficult to form a homogeneous film because the degree of heating of the sample differs between on the central portion and on the edge portion. Therefore, this method requires a large electric furnace capable of precise temperature control. In addition, it is difficult to prepare a superconducting film having good characteristics with high reproducibility because there exist not a little different phases.
As a method that improves MOD described above by which crystallites in the calcined film exert no influence on the fired structure, TFA-MOD is developed. TFA-MOD was first reported by Gupta et al. in 1988. At that time, the purity of solutions was presumably low owing to the effect of starting materials, so TFA-MOD did not provide any particularly outstanding characteristics or reproducibility like other MOD methods. Afterwards, McIntyre et al. improved TFA-MOD, and a superconducting critical current density (JC) exceeding 1 MA/cm2 at 77 K and 0 T was realized.
Although TFA-MOD is a type of MOD, crystallites in the calcined film exert no influence on orientation of the fired structure. TEM observation shows that a large number of nanocrystallites exist in the cross-section of the calcined film, but all these nanocrystallites disappear after firing, so a biaxially oriented structure is formed with high reproducibility (T. Araki and I. Hirabayashi, Supercond. Sci. Technol., 16, R71 (2003)). In TFA-MOD, therefore, unlike in normal MOD, carbon which is harmful to superconducting characteristics can be expelled almost completely by calcining continued over 10 hours or more, so a superconducting film having good characteristics can be prepared with high reproducibility (T. Araki, Cryogenics, 41, 675 (2002)). Initially, the growth mechanism during firing was unknown. However, it is recently found that a quasi-liquid network is formed due to mixing of fluorine, and this eliminates crystallites in the calcined film. This reveals in principle that TFA-MOD provides high reproducibility and good characteristics which cannot be realized by any normal MOD (T. Araki et al., J. Appl. Phys., 92, 3318 (2002)). Since a chemical equilibrium reaction deeply contributes to the growth through the formation of the quasi-liquid network, a slight amount of fluorine remains in the film, which is a feature of TFA-MOD. However, it is also found that this slight amount of residual fluorine does not deteriorate the superconducting characteristics.
It is thought that one of the causes of the excellent characteristics and amazing reproducibility of the Y-based superconducting film provided by TFA-MOD is reduction of a/b-axis-oriented grains. The a/b-axis-oriented grains have such a structure that the c-axis-oriented grain, in which a superconducting current flows in a direction parallel to the plane, is fallen down sideways. Since in the a/b-axis-oriented grains a superconducting current primarily flows in a direction perpendicular to the plane, the particular grains significantly degrade the superconducting current in the direction parallel to the plane, and thus, degrade the superconducting characteristics. The following three points are presently regarded as main causes of the formation of the a/b-axis-oriented grains.
(1) The firing conditions (oxygen partial pressure and temperature) are not optimum.
(2) Impurities exist in the solution.
(3) The lattice constants of the c-axis-oriented grains and single-crystal substrate are mismatched.
For the factor (1), Hammond and Bormann reported that there are optimum conditions independent of the manufacturing method of a superconductor (R. H. Hammond and R. Bormann, Physica C 162-164, 703 (1989)). According to this report, whenever the oxygen partial pressure during firing reduces by half, the optimum firing temperature decreases by about 25° C. The optimum firing conditions of MOD and TFA-MOD are the same as, e.g., PLD.
For the factor (2), experiments have shown that when an impurity amount reduces, the ratio of the c-axis-oriented grains increases. It is also disclosed that the superconducting characteristics greatly improve as well (Japanese Patent No. 3,556,586).
For the factor (3), a superconductor presumably has the a-, b-, and c-axis lengths unique to the material. However, when a thin film is formed on a single-crystal substrate, the film epitaxially grows in a strained state in accordance with the lattice constant of the substrate for the reason described below. While the film thickness of a superconductor obtained by this method is 0.1 to 10 μm, the thickness of a single-crystal substrate is about 0.4 to 1.0 mm, i.e., approximately 1,000 times the film thickness of the superconductor, so the substrate is very strong. Therefore, if the lattice constants of the thin film and substrate are different, the superconducting film presumably grows in a strained state. Immediately above the single-crystal substrate, substantially the same lattice constant as that of the substrate is probably observed. When a thin film about 0.1 μm thick is formed and phase identification is performed by XRD, a value close to the lattice constant of the substrate is observed. However, if the film thickness increases, the lattice constant of the film presumably approaches the intrinsic lattice constant of the superconductor. Whether the a/b-axis-oriented grains or c-axis-oriented grains grow more easily is determined by the lattice constant of the single-crystal substrate.
When TFA-MOD is used to prepare YBCO superconducting films having film thicknesses of 150 to 300 nm on four types of single-crystal substrates, i.e., LaAlO3, NdGdO3, SrTiO3 and CeO2/YSZ, peak intensity ratios of the a/b-axis-oriented grains determined by XRD measurement decrease in the order of NdGdO3, LaAlO3, SrTiO3 and CeO2/YSZ, regardless of the film thickness. That is, when the CeO2/YSZ substrate is used, a superconducting film with a highest JC value, i.e., 11 MA/cm2 (77 K, 0 T) for a film thickness of 0.22 μm, is provided (T. Araki and I. Hirabayashi, Supercond. Sci. Technol., 16, R71 (2003)). In order to increase the c-axis orientation ratio, it is desirable to change the intrinsic lattice constant of the substrate or superconductor. However, the lattice constant of the substrate cannot be continuously changed. This is so because materials usable as the single-crystal substrate are limited, and their lattice constants are discontinuous values as well.
Accordingly, it is being demanded to increase the c-axis-oriented grain ratio by adjusting the axial length of the superconductor. As described above, Y element in the YBCO superconductor can be substituted with a specific Ln series element. The Ln series elements exhibit lanthanoide contraction by which the ion radius contracts in accordance with the atomic number. This presently makes it difficult to measure the a-, b-, and c-axis lengths of each Ln-based superconductor, but the axial lengths of the individual superconductors may be substantially different. It is expected to prepare a superconductor with a high ratio of c-axis-oriented grains by mixing two or more types of Ln-based superconductors in accordance with the substrate used.
As described above, La, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and the like are known as Ln series elements which show superconducting characteristics when substituting Y. However, it is difficult to apply TFA-MOD to some of these Ln series elements. In the case of La, Nd, or Sm with a smaller atomic number, when a methanol solution of trifluoroacetate salt is to be prepared, an esterification reaction occurs to allow decomposition of the salt. If refining is performed under conditions by which no esterification reaction occurs, good superconducting characteristics cannot be obtained because of large amounts of impurities. In the case of Yb with a larger atomic number, the solubility thereof is extremely low, so it is impossible to prepare any solution from which a film with a practical thickness can be obtained.
In the case of Gd, Tb, Dy, Ho, Er, or Tm, similar to yttrium trifluoroacetate, a high-purity methanol solution can be prepared by the SIG (Solvent-Into-Gel) method (T. Araki et al., Supercond. Sci. Technol., 14, L21 (2001), Japanese Patent No. 3,556,586), and a superconductor containing a single Ln series element can be provided. The critical current density (JC) of each of these superconductors is 3 to 4 MA/cm2 (77 K, 0 T), which is a sufficiently high value in terms of usefulness (T. Iguchi et al., Physica C 392-396 900 (2003); T. Iguchi et al., Superconduct. Sci. Technol., 15, 1415 (2002)). However, this JC value is about half that of the YBCO-based superconductor, i.e., 7 MA/cm2 (77 K, 0 T). It was expected that mixing the individual Ln-based solutions would provide a superconductor in which the a-, b-, and c-axis lengths of each superconductor containing a single Ln series element can be freely adjusted within a certain range. However, any superconductor prepared by mixing the individual Ln-based solutions exhibited a low JC value. More specifically, a superconductor prepared by mixing the individual Ln-based solutions at a ratio of 1:1 exhibited a lowered JC value of about 1 MA/cm2 (77 K, 0 T). The purities of lanthanoide acetates used as Ln sources were 97% to 98%, so it is likely that impurities increased the ratio of the a/b-axis-oriented grains and deteriorated the characteristics.